When I get like that I trick myself (unless I'm in a major slump like I've just come out of). But when I'm just feeling lazy, I'll tell myself I'm just going to do something short, like a half session or half the sets or just go do what I can for 15 or 20 minutes or something.
Nine times out of ten I end up feeling better once I start moving and do it all anyway.
The tenth time at least you've done a small workout, which is much better than nothing at all.